The present invention relates to a secure transmission system in which a plurality of pieces of secure communication equipment perform secure communication between them over a transmission line. The invention also pertains to such secure communication equipment.
In a conventional transmission system for secure communication pieces of secure communication equipment 100A and 100B connected to a transmission line 10L, as shown in FIG. 1, each transmit information in an encrypted form to the other and decode an encrypted signal received therefrom to obtain information. For example, in the secure communication equipment 100A, an information signal A to be transmitted is encrypted by a scrambling circuit 11A and is then provided onto the transmission line 10L from a transmitting circuit 12A. The scrambling circuit 11A encrypts the information signal A, using a fixed random pattern K1 (an encrypting key) which is available from a pseudo random pattern generator 15A. The encrypted signal A' is transmitted by the transmitting circuit 12A to the communication equipment 100B over the transmission line 10L. On the other hand, a signal B' received by a receiving circuit 13A from the communication equipment 100B through the transmission line 10L is decoded by a descrambling circuit 14A using a fixed random pattern K2 (decoding key) available from a pseudo random pattern generator 16A, whereby an information signal B from the communication equipment 100B is obtained.
With such a conventional secure transmission system as shown in FIG. 1, the use of very long random patterns as those K1 and K2 for encryption and the complication of conversions, intended for enhanced security of data on the transmission line, will enlarge the circuit scales of the scrambling circuits 11A, 11B and the descrambling circuits 14A, 14B and will raise the cost of the entire system. In addition, since the random patterns K1 and K2 are fixed, it is difficult to completely preclude the possibility of transmitted information being decoded by a third party. For instance, in the case where pieces of secure communication equipment of the same construction as described above are connected to the transmission line 10L in addition to the pieces of communication equipment 100A and 100B, security of the transmission of information between them will degrade, if their scrambling and descrambling circuits are formed by circuits of the same specifications and the same conversion rules and conversion periods are employed.